All variable valve lift and timing mechanisms are designed with the objective of improving volumetric efficiency. A review of the tenets of cam design is presented to identify ideal valve operation and function across a range of engine rotation speeds. This summary also identifies the inherent compromises of fixed cam lobe design that must balance engine economy with output power. Using a “gold standard”, for purposes of objective comparison, all devices that claim variable valve operation should be assessed in their ability to emulate ideal characteristics of operation and function over a range of engine speeds. The competing devices should then be judged by cost verses improvement to volumetric efficiency. The expense of integrating a variable valve operation device into production will include: the total number of device components, the sophistication level of material processing, the number of labor hours for assembly, and the dimension tolerances required for the components to meet design specifications.
A set of “pie” diagrams are shown in FIGS. 1a through 1e, for the intake valves and 2a through 2e for the exhaust valves beginning on page 39. The diagrams graphically depict the duration period of valve opening measured in degrees of crank rotation. Each diagram set, e.g. 3a & 3b, show volumetric efficient, intake and exhaust valve opening duration envelopes for each increasing stage of engine speed. FIG. 1a is a “pie” diagram of an intake valve that opens at Top Dead Center (T.D.C. 0°) and closes at Bottom Dead Center (B.D.C. 180°). The diagram depicts a valve opening duration of one hundred eighty degrees (180°) degrees. If not ideal, it is close to an ideal intake valve opening duration for idle speed. An intake valve opening duration envelope for medium cruise speed is shown in FIG. 1c. The valve opens eight degrees (8°) before T.D.C. and closes seventeen degrees (17°) after B.D.C; an opening period of two hundred five degrees (205°). 8°+17°+180°=205°. In FIG. 1e the valve opens fifteen degrees (15°) before T.D.C. and closes forty-five degrees (45°) after B.D.C.; two hundred forty degrees (240°) of crank rotation. 15°+180°+45°=240°. This is close to an optimum duration for high speed engine operation.
The “M.P.” represents “Mid-Point” in the “Pie” Diagrams of FIGS. 1a–1e through 6a–6e covering pages 39, 40 and 41. The Mid-Point is the arithmetical half-way mark of the valve's opening duration relative to the degree period of crank shaft rotation. In a conventional camshaft arrangement the Mid-Point of the valve opening duration occurs when the valve achieves maximum lift off the valve seat at the half way point of rotation over the cam lobe. In the present invention, the Mid-Point of valve opening duration is not the rotation point of maximum valve lift. The Mid-Point is used as a mark of reference to compare the amount and direction of the timing shift between Two (2) valve opening duration envelopes.
In sequence, from FIGS. 1a to 1e, there is an asymmetrical expansion of the intake valve opening envelope into the exhaust and compression strokes of the engine. In the expanding progression of this graphical surrogate for the intake valve opening duration, there is greater encroachment by the envelope into the compression stroke than into the exhaust stroke. It is important to identify the degree amount of shift in the Mid-Point with respect to T.D.C.
In FIG. 1a the Mid-Point of the duration envelope is ninety degrees (90°). In FIG. 1b, location of the Mid-Point is close to ninety-four degrees (94°). The Mid-Point of the valve opening duration envelope continues to shift for FIGS. 1c and 1d. FIG. 1e shows the Mid-Point of the envelope has been retarded a total of fifteen degrees (15°); starting at ninety degrees (90°), FIG. 1a, and shifting to one hundred five degrees (105°) past T.D.C.
FIGS. 2a through 2e show the incremental expansion of a set of envelopes for the period of exhaust valve opening duration. As the exhaust envelope expands, its Mid-Point will shift direction opposite to the Mid-Point shift of the intake envelopes. In FIG. 2a, the Mid-Point is two hundred seventy degrees (270°) past T.D.C. In FIG. 2b, the Mid-Point of the duration envelope is close to two hundred sixty-six degrees (266°) past T.D.C. The duration envelope Mid-Point continues to shift through FIGS. 2c and 2d. In FIG. 2e the Mid-point of the exhaust valve envelope has shifted to two hundred fifty-five degrees (255°) after T.D.C. Following in a sequence, FIGS. 2a through 2e show an advance of the Mid-Point of the exhaust envelopes by fifteen degrees (15°).
Up to a point of diminishing returns, a greater level of valve lift from the valve seat poses less restriction to the flow of air or exhaust gases. FIGS. 3a through 3e, and FIGS. 4a through 4e, on page 40, show a series of “pie” diagrams with a profile graph of the valve lift adjacent to the duration envelopes. The lift profiles display the actual and relative levels of valve lift. The lift profiles also show the valve lift in relation to the valve opening envelope and the rotational position of the crankshaft.
Compare the lift profiles as an intake and exhaust set over FIGS. 3a and 4a through 3e and 4e. The amount of exhaust valve lift is usually two-thirds (⅔) of the intake valve lift. The exhaust valves open only to a level necessary to achieve substantial evacuation of the exhaust gases and not produce a back pressure on the ascending piston during the exhaust stroke.
FIGS. 3a through 3e and 4a through 4e show a symmetrical set of lift profiles across each of the duration envelopes. Maximum valve lift and the Mid-Point of each envelope is at the half way mark through the valve opening duration. FIGS. 5a through 5e, and 6a through 6e, on page 41, have asymmetrical valve lift profiles that will further improve volumetric efficiency. The intake valve lift profiles of FIGS. 5a through 5e, show the valve opening slowly and reaching maximum lift near the end of the duration envelope. Irrespective of engine speed, the optimum point for maximum intake valve lift occurs as the descending piston approaches B.D.C., where cylinder volume is not expanding by an appreciable amount.
During the intake stroke, the maximum piston velocity occurs when the crankshaft is at ninety degrees (90°) and the piston is half-way to B.D.C. At this crank position, the velocity of the air entering the cylinder lags behind the velocity of the piston. After the piston passes ninety degrees (90°), piston speed will decrease during its descent toward bottom dead center. During the second phase of the intake stroke, the increasing velocity of the air column entering the cylinder will exceeds the decreasing piston velocity. Efficient cylinder filling is optimized when the intake valve reaches its maximum lift at, or near, bottom dead center.
A variable valve device that emulates the duration envelopes and the intake lift profiles of FIGS. 5a through 5e, will improve the engine efficiency for several reasons. Although the compressed valve spring returns most of its stored energy to the rotating system, the longer period of cam rotation to maximum intake valve lift will reduce the power consumed internally to overcome spring resistance and reduce the losses from component friction.
With an increase in the engine rotation speed, the level of intake valve lift can be chosen to slightly restrict airflow into the cylinder. This restriction will increase the air velocity around the intake valve. The longer rotation period of the cam and the gradual rise of intake valves to maximum lift, provides an opportunity to use the high velocity airflow to create a more uniform dispersion of smaller fuel droplets within the cylinder.
At high levels of engine speed, the use of an asymmetrical lift profile also addresses the problem of time lost at the start of the valve opening envelope. As the intake valve opens there is a period of lost intake duration time due to the inertia of the stationary air column entering the cylinder. As engine speed increases to the high end of the r.p.m bandwidth, this time loss due to air column inertia poses a increasing detriment to volumetric efficiency. The problem is partially rectified by expanding the duration period of valve opening further into the exhaust and compression strokes to gain additional time to fill the cylinder.
Refer to the high speed “pie” diagrams of FIGS. 2d and 2e. A conventional camshaft, with a “2d” or “2e” type of valve envelope, will sacrifice a portion of the compression stroke as the intake valve remains open past B.D.C., and the piston has started its upward motion. One of the compromises of a conventional fixed cam is the loss of actual compression ratio at all engine speeds to assure an adequate filling of the cylinder at high r.p.m.
The Variable Geometry Camshaft expands the intake duration envelope into the compression stroke only as increasing engine speed would warrant the intrusion. The maximum available compression ratio can be used at idle, and the lower speeds of engine operation. During the operation of a V.G.C. engine at cruise levels and above, it is expected that the asymmetrical intake-valve lift profile of the invention will mitigate the amount of valve opening encroachment into the compression stroke. Over the bandwidth of engine operation speeds there is a less sacrifice of the compression ratio. The Variable Geometry Camshaft will continuously adjust the close of intake valves to improve volumetric efficiency and minimize the loss of actual compression at the higher levels of engine speed.
An optimum set of duration envelopes and valve lift profiles for the exhaust valves are presented in FIGS. 6a through 6e. The profiles show the exhaust valve opening quickly, maintaining a nearly constant level and then tapering off to closure. There is a benefit in opening the exhaust valve to maximum lift during the first phase of exhaust stroke to rapidly evacuate the gases from the cylinder. By minimizing cylinder pressure early with a wide open exhaust valve, less power is internally consumed in overcoming the back pressure from forcing waste gases out of the cylinder during the exhaust stroke. Unlike the intake valve, the rotation of the cam to engage the camfollower in close proximity to the fulcrum, offers a mechanical resistance to rapid lift of the exhaust valve. Fortunately, the amount of exhaust valve lift is generally less than the amount for the intake valve. It is also expected that due to the exhaust lift profile's gradual reduction of valve opening to closure, a spring with a lower coefficient of compression resistance can used without developing valve seat bounce at high r.p.m.'s.
With increasing engine speed, greater exhaust valve lift over an expanded opening duration is also used to offset the reduction in available time to evacuate the cylinder. For operation at idle speed, it is aimless to open the exhaust valve to a maximum level of lift. If the cam lobe is not required to overcome the greater resistance from full travel of the exhaust valve spring, there is a gain in economy due to the reduction of internal resistance.
To achieve nearly complete cylinder evacuation at higher levels of engine speed, the invention increases the point of exhaust valve opening before the piston reaches bottom dead center on the power stroke. The escaping exhaust gases produce a reactive force on the downward moving piston that is often referred to as the “kick.” A rapid opening of the exhaust valve will maximize the amount of additional reactive force or “kick” on the piston before B.D.C.
A fixed camshaft requires compromises in the selection of lobe dimensions that limit volumetric efficiency to a narrow range of engine speeds. A design for a fixed cam lobe must balance the competing interests of economy and the ability to obtain high output power on demand. The Variable Geometry Camshaft provides an alternative to the inherent compromises of fixed cam valve timing, the level of valve lift and the extent of valve opening duration.
FIGS. 7a through 7f, show a cam lobe with a Thirty degree (30°) slope with respect to a centerline that extends through the camshaft axis to lobe apex. In FIGS. 7a through 7f, the cam heel and valve lifter are separated by a pre-determined distance. This separation distance is close to half the maximum rise of the cam lobe. Following FIGS. 7a through 7f, the cam lobe rotates and the lifter reaches its maximum height in FIG. 7c. The maximum potential for valve lift is reduced by the separation distance between cam heel and lifter. In FIG. 7e the rotating cam loses its contact with the valve lifter. The cam's rotation, progressing from FIGS. 7a through 7e, will operate the lifter (and valve) over a ninety degree (90°) period. This is equivalent to one hundred eighty degrees (180°) of crankshaft rotation.
In FIGS. 8a through 8f, the space between the cam heel and the lifter surface is reduced from the amount shown in FIGS. 7a through 7e. In FIG. 8a the cam begins to exert a force on the lifter earlier in the cam's rotation with respect to FIG. 7a. With a reduced separation distance between the cam heel and the lifter, the lifter's maximum rise is now greater than the amount shown in FIG. 7c. With further rotation the cam lobe loses its contact with the valve lifter as shown in FIG. 8e. In FIGS. 8a through 8e, the duration period of cam lobe to lifter contact is one hundred five degrees (105°) of cam rotation; equivalent to two hundred ten degrees (210°) of crankshaft rotation.
In FIGS. 9a through 9f, the separation distance between the cam heel and lifter surface is now negligible. The point of cam lobe contact with the lifter (FIG. 9a) is now earlier than shown in either FIG. 7a or 8a. The increased rotational period of contact between cam lobe and valve lifter is one hundred twenty degrees (120°); or two hundred forty degrees (240°) of crankshaft rotation. With a minimum spacing between lifter and cam heel, the full height of the cam lobe is now impressed upon the lifter as shown in FIG. 9c. 
The patent of Griffiths (U.S. Pat. No. 6,189,497) presented this method of changing valve lift and valve opening duration through the limited movement of the cam axis around a d-rive gear as shown in FIG. 10. The movement of the cam axis over a small arc is essentially a linear motion similar to the position change of the cam axis shown in FIGS. 7, 8 and 9.
The patent of Griffiths is an improvement over the limitations of a fixed dimension camshaft. This patent is, however, restricted in delivering an optimum level of volumetric efficiency. Notice that maximum valve lift occurs at the Mid-Point of the duration envelope. This method of cam to lifter engagement will produce lift profiles similar to FIGS. 3a through 3e and 4a through 4e. The previous patent is also limited by the length of the cam axis movement. This arc length is less than the-total amount of cam lobe rise. This limit on the cam rotation to asymmetrically shift the Mid-Point of an expanding or contracting valve opening duration envelope is therefore restricted. The limited cam rotation would require the components to meet high standards of dimension tolerance to achieve uniformity in valve operation.
FIG. 10 is a reprint from Griffiths that shows major device components. In FIG. 10, the ramp shaft is housed in the camshaft location within the engine and can rotate two hundred seventy degrees (270°) between stop positions. The graduated rise of the ramp has pushed the suspension bracket to rotate the cam four to eight degrees (4–8°) clock-wise around the drive gear. The cam heel has compressed the telescoping lifter. With cam rotation, the full amount of lobe lift is transferred to valve operation. As the ramp shaft is rotated clockwise, the return spring pulls the suspension bracket and cam away from the telescoping lifter. As the cam rotates to its contact point it must first compress the lifter and make up the distance that the cam axis has been displaced. At a rotation point where the valve lifter is fully compressed, further rotation of the cam will then begin to operate the valve. A change in the separation distance between the Two (2) piece lifter changes the amount of valve lift and opening duration.
The U.S. Pat. No. 6,189,497 provided the basic concept of permitting the cam axis to rotate in a limited arc to change the geometry of its interation with the valve train. The present invention offers a method of increasing the length of the arc of cam axis rotation. The present invention also coordinates an asymmtrical valve lift to suit the requirements of intake or exhaust operation. Moreover, the present invention offers a unique method of continuous mechanical control of all of the functions of variable valve operation.